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Quantum thermal machines make use of non-classical thermodynamic resources,one of which is interactions between elements of the quantum working medium. Inthis paper, we examine the performance of a quasi-static quantum Otto enginebased on two spins of arbitrary magnitudes subject to an external magneticfield and coupled via an isotropic Heisenberg exchange interaction. It has beenearlier shown that the said interaction provides an enhancement of cycleefficiency for two spin-1/2 particles, with an upper bound which is tighterthan the Carnot efficiency. However, the necessary conditions governing engineperformance and the relevant upper bound for efficiency are unknown for thegeneral case of arbitrary spin magnitudes. Analyzing extreme-case scenarios, weformulate heuristics to infer the necessary conditions for an engine withuncoupled as well as coupled spins model. These conditions lead us to aconnection between performance of quantum heat engines and the notion ofmajorization. Further, the study of complete Otto cycles inherent in theaverage cycle also yields interesting insights into the average performance.

Quantum heat engines with complex working media, complete Otto cycles and heuristics